1. Field of the Invention
The present invention relates to cables for the transmission of information and, more particularly, to transmission cables having both optical fiber and metallic conductor components.
2. Description of the Prior Art
Optical fiber and copper cables have been used for the past several years to transmit information (voice, data, video, etc.) over relatively long distances. The optical fiber transmission medium is composed of dielectric hair-thin optical fibers. Copper cables have a transmission medium composed of insulated high grade copper conductors.
Optical fiber cable structure families which are currently being used are loose tube, slotted core, monotube and tight tube. In a loose tube structure, several gel filled buffer tubes containing optical fibers are stranded around a central strength member. The slotted core structure has optical fibers precisely placed in gel filled channels or slots. The channels are symmetrical and form a helical path along the longitudinal axis of the cable. A strength member is located in center of the slotted core cable structure. In the monotube structure, all of the optical fibers are in a single, centrally located, gel filled buffer tube. Finally, in the tight buffer tube construction, protective layers are applied in direct contact with each optical fiber so there is no fiber overlength.
All the above cable structures may incorporate various sheath constructions (armored, nonarmored, high strength, self-supporting, flame retardant, etc.) depending on the application and environment.
Optical fiber cables are slowly taking the place of conventional copper cables due to their high bit-rate capacity, small size and light weight. The only concern in moving exclusively to an all-fiber network is in supplying power to the send/receive hardware at the remote terminals, pedestals, etc. Conventional copper cables currently have enough capacity to carry an adequate amount of power to energize the hardware. The power can be obtained by tapping off the neighborhood or household electrical lines, but then localized power outages will also cause telephone outages. There are currently back-up generators which can continually supply electrical power to the copper cables so that the telephone service will not be interrupted. Battery back-ups can also be used, but they are not suitable during long-term (greater than 4-6 hours) power outages.
There currently is not a very economical way to slowly transition from copper cables to optical fiber cables. The costs involved in removing the existing copper cables and installing optical fiber cables are quite high.